1. Technical Field
The present invention relates to a riding-type ground working vehicle including left and right wheels that are main drive wheels independently driven to travel by two respective traction motors, steering control wheels that are provided to be separated in a longitudinal direction with respect to the left and right wheels, and a working machine that is driven to perform ground work.
2. Related Art
Ground working vehicles have been conventionally known, including working machines that are driven to perform ground work such as lawn mowing work and tilling. Further, in relation to such ground working vehicles, there are considered electric ground working vehicles, which include left and right wheels that are main drive wheels which are independently driven to travel by respective motors such as electric motors and hydraulic motors, and steering control wheels such as caster wheels.
For example, the working vehicles include a lawn mowing vehicle loaded with a lawnmower as a working machine and capable of self-powered travel, on which a worker rides to perform traveling and mowing operations onboard. This is called a riding lawnmower vehicle. As the lawnmower, there is used, for example, a lawn mower rotary tool or the like.
A riding lawnmower vehicle is used almost exclusively for so-called “off-road” usage in a garden or the like, and moves over the surface of the ground for lawn mowing work.
For example, National Publication of International Patent Application No. 2006-507789 discloses a hybrid power apparatus that has mounted thereon a device that integrates an engine and an electricity generator which connects a rotor to an engine shaft of an internal combustion engine. It is noted that in a lawnmower that is exemplified as a power apparatus, independent electric motors are respectively linked to a plurality of drive wheels so that each drive wheel can be controlled at independently variable speed, thereby allowing smooth performance of starting, stopping, speed changing, direction changing, and turning of the lawnmower. As an example of turning executed by independent speed changes of the drive wheels, there is mentioned an apparatus in which the left and right rear wheels are linked with respective electric motors.
U.S. Pat. No. 7,017,327 discloses, as a hybrid lawnmower, a configuration in which electric power produced by an alternator connected to an engine disposed at the front is used to drive a deck motor for lawnmower blade driving, left and right wheel motors for driving independently controlled left and right rear wheels, and steering motors that steer left and right front wheels over a range of approximately 180 degrees about an axle. In this case, to turn the lawnmower, the speed difference between the left and right rear wheels is calculated based on input from a steering control section to control the wheel motors, and a steering signal is supplied to the steering motors to control the positions of the left and right front wheels. It is noted that, as a result, the lawnmower can be turned without steering the left and right rear wheels. Related art literature that relates to the present invention includes, in addition to the specifications of National Publication of International Patent Application No. 2006-507789 and U.S. Pat. No. 7,017,327, Japanese Patent Laid-Open Publication No. 2005-184911 and Japanese Patent No. 3853907.
In the case of each of the riding lawnmower vehicles described in National Publication of International Patent Application No. 2006-507789 and U.S. Pat. No. 7,017,327, the left and right rear wheels are independently driven by motors, and it is conceivable that when traveling on a sloping surface (e.g., traveling on contour lines across a sloping surface), the force in the direction different from the traveling direction acts on each of the wheels based on the force of gravity. In this case, there is the possibility that the steering control wheels fail to turn in the direction desired by a driver, or that the wheels slide sideways, and it is likely to be difficult for the vehicle to travel in a straight line direction or a turning direction which is the direction desired by the driver. Especially when the steering control wheels are caster wheels which freely rotate around the vertical axis, making the vehicle travel in a desired direction is likely to be more difficult.
In contrast, Japanese Patent Laid-Open Publication No. 2005-184911 describes a wheel-independently-driving electric automobile which independently drives wheels by individual electric motors. In the electric automobile, the output target value of each of the electric motors is calculated based on an accelerator pedal depression amount and a vehicle speed, the target yaw rate is obtained based on a steering angle and a vehicle speed, and the output target value of each of the electric motors is corrected and output so as to eliminate the deviation between the target yaw rate and the detected yaw rate, which is the yaw rate detection value which is the turning angular velocity about the vertical axis passing through the center of gravity of the electric automobile.
Further, Japanese Patent No. 3853907 describes a drive control apparatus which gives an instruction relating to output to each of a plurality of electric motors for driving left and right drive wheels of an electric automobile. The drive control apparatus includes section which temporarily defines an output instruction based on an acceleration or deceleration request, and section which corrects the instruction before it is given to the electric motor, based on a plurality of state quantities including the detected value of the yaw rate of the vehicle body, and the section corrects the torque command so that a positive or negative moment is produced from the lateral acceleration, the yaw rate detection value, and the like in accordance with a positive or negative sign of the indication showing the yaw movement direction of the vehicle. Further, the moment of the vehicle is determined based on the difference between the plurality of state quantities which are fed back and the target state quantity obtained by using the detected value of the steering angle, and the torque is distributed to the left and right drive wheels in accordance with the determined moment of the vehicle.
In each of the configurations described in Japanese Patent Laid-Open Publication No. 2005-184911 and publication of registered Japanese Patent No. 3853907 as mentioned above, there is the possibility a driver can travel in the straight line direction and the turning direction which is the direction desired by the driver when the vehicle travels on the sloping surface, although the vehicle is an electric automobile instead of a riding lawnmower vehicle. However, in these configurations, it is difficult to set independent corrected values for the left and right drive wheels, and it is difficult to freely set the behavioral characteristics of the vehicle; that is, a vehicle speed and the like, at the time of the control operation of correcting the rotational speed instruction for the electric motors based on the deviation between the detected values and the target values of the yaw rate. For example, in the configuration of Japanese Patent Laid-Open Publication No. 2005-184911, the same output target is calculated for each of the electric motors for driving left and right wheels, and the output target is corrected based on the deviation relating to the yaw rate, but the concrete correction method is not described. Further, in the configuration of Japanese Patent No. 3853907, the same output instruction is temporarily defined in each of the electric motors, and the torque instruction is corrected based on the yaw rate detection values and the like, but the yaw rate target value is not obtained, and it is difficult to freely set the behavioral characteristics of the vehicle at the time of control operation based on the deviation between the detected value and the target value of the yaw rate. Further, such a disadvantage is likely to occur similarly in the case of riding-type ground working vehicles having other working machines besides the riding lawnmower vehicle having a lawnmower as described above.